Gas-and-Steam Combined-Cycle Power Plant

ABSTRACT

The present disclosure relates to power plants. Various embodiments thereof may include a method for operating a gas-and-steam combined-cycle power plant. For example, some embodiments may include a method for operating a gas-and-steam combined-cycle power plant including: providing exhaust gas from a gas turbine to a steam generator; generating steam by means of the exhaust gas; driving a generator with the steam via a turbine installation to provide an electric current; removing the exhaust gas from the steam generator; and using at least a portion of heat contained in the exhaust gas downstream from the steam generator to affect an endothermic chemical reaction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2016/072847 filed Sep. 26, 2016, which designatesthe United States of America, and claims priority to DE Application No.10 2015 219 403.5 filed Oct. 7, 2015, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to power plants. Various embodimentsthereof may include a method for operating a gas-and-steamcombined-cycle power plant.

BACKGROUND

Gas-and-steam combined-cycle power plants may be known as COGAS powerplants. The gas-and-steam power plant is also referred to as acombined-cycle power plant, and typically comprises at least one turbineinstallation, at least one generator that can be driven by the turbineinstallation, for providing electric current, and at least one gasturbine. When the generator is driven by the turbine installation, thegenerator can convert mechanical energy into electrical energy, orelectric current, and provide this electrical energy, or the electriccurrent. The electric current can then be fed, for example, into anelectricity grid.

The gas turbine provides exhaust gas, by means of which hot steam may begenerated. For example, a fuel, such as a gaseous fuel for example,natural gas, is supplied to the gas turbine, the fuel then burned bymeans of the gas turbine. In particular, in addition to the fuel, oxygenor air is supplied to the gas turbine, such that a fuel-air mixture isproduced from the air and the fuel. This fuel-air mixture is burned,resulting in exhaust gas of the gas turbine. By means of the exhaustgas, a fluid, e.g. water, is heated and thereby evaporated, resulting inhot steam. This means that the hot steam is generated by means of theexhaust gas of the gas turbine in such a manner that a fluid such as,for example, water, is evaporated by means of the hot exhaust gas of thegas turbine.

The steam is then supplied to the turbine installation, such that theturbine installation is driven by means of the steam. As alreadydescribed, the generator is driven via the turbine installation, or bymeans of the turbine installation. The gas-and-steam combined-cyclepower plant is a power plant in which the principles of a gas-turbinepower plant and a steam power plant are combined. The gas turbine, orits exhaust gas, serves in this case as a heat source for a downstreamsteam generator, by means of which the steam for the turbineinstallation, for driving the turbine installation, is generated. Theturbine installation is thus realized as a steam turbine.

This means that the gas turbine provides its exhaust gas, which issupplied to the steam generator. Thus, by means of the exhaust gassupplied to the steam generator, and by means of the steam generator,hot steam is generated, by means of which the turbine installation isdriven and, via the turbine installation, the generator is driven, forthe purpose of providing electric current. In addition, the exhaust gassupplied to the steam generator is removed again, at least in part.

It has been found that such a gas-and-steam combined-cycle power plant(COGAS power plant) must be switched off in response to the electricitydemand, such that the generator does not provide an electric currentand, for example, is not driven, and such that no current is fed intothe electricity grid by means of the COGAS power plant. Owing to theswitch-off, the gas-and-steam combined-cycle power plant can cool down,whereupon a renewed start-up, or ramping-up, of the gas-and-steamcombined-cycle power plant requires a particularly long time and aparticularly high energy demand.

For this reason, the gas-and-steam combined-cycle power plant is usuallykept warm during the period in which the gas-and-steam combined-cyclepower plant is switched off. In this case, the gas-and-steamcombined-cycle power plant is kept warm by means of steam. This steamfor retaining warmth is usually generated by means of a boiler, e.g. agas boiler. The boiler evaporates a fluid such as, for example, water.The steam generated by means of the boiler is routed at least through apart of the gas-and-steam combined-cycle power plant, to keep the latterwarm, or heat it. The gas-and-steam combined-cycle power plant, afterhaving been switched off, can then be started in a warm-start operation,since the gas-and-steam combined-cycle power plant then already has asufficiently high temperature at which it can be started. Nevertheless,as the time during which the gas-and-steam combined-cycle power plant isswitched off increases, an increasing quantity of steam is required tokeep the gas-and-steam combined-cycle power plant warm, or to heat it,since it cools down gradually.

SUMMARY

The teachings of the present disclosure may be embodied in methods thatoffer particularly efficient operation. For example, a method foroperating a gas-and-steam combined-cycle power plant (10) in whichexhaust gas is provided by a gas turbine (12) and is supplied to a steamgenerator (20), wherein hot steam is generated by means of the exhaustgas supplied to the steam generator (20) and by means of the steamgenerator (20), which steam is used to drive at least one generator(30), via at least one turbine installation (22), for the purpose ofproviding electric current, and wherein the exhaust gas supplied to thesteam generator (20) is removed from the steam generator (20), mayinclude at least a portion of heat contained in the exhaust gasdownstream from the steam generator (20) is used to effect anendothermic chemical reaction.

In some embodiments, at least the portion of the heat contained in theexhaust gas downstream from the steam generator (20) is transferred, viaa heat exchanger (38), to educts of the endothermic chemical reaction.

Some embodiments include branching-off at least a portion of the steamgenerated by means of the steam generator (20) and storing thebranched-off steam in a steam accumulator (34); removing at least aportion of the steam, stored in the steam accumulator (34), from thesteam accumulator (34); heating the steam removed from the steamaccumulator (34) by means of heat that is released in an exothermicchemical reaction; and routing the heated steam to the turbineinstallation (22), which is driven by means of the supplied heatedsteam.

In some embodiments, products of the endothermic chemical reaction areused as educts of the exothermic chemical reaction.

In some embodiments, the heated steam for driving the turbineinstallation (22) is supplied to the turbine installation (22), in orderto ramp up the gas-and-steam combined-cycle power plant (10) from afirst load range into a second load range that is higher than the firstload range.

In some embodiments, the endothermic chemical reaction is effected inthe second load range.

Some embodiments may include a gas-and-steam combined-cycle power plant(10) that executes a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features, and details are disclosed by the followingdescription of an exemplary embodiment and with reference to thedrawing. The features and feature combinations mentioned in thedescription and the features and feature combinations mentioned in thefollowing description of the FIGURE and/or shown alone in the singleFIGURE can applied, not only in the respectively specified combination,but also in other combinations or singly, without departure from thescope of the invention.

The drawing, in the single FIGURE, shows a schematic representation of agas-and-steam combined-cycle power plant, in which a thermochemical heataccumulator is used to realize a particularly high efficiency accordingto the teachings of the present disclosure.

DETAILED DESCRIPTION

Particularly efficient operation can be realized in embodiments whereinat least a portion of heat contained in the exhaust gas of the gasturbine downstream from the steam generator is used to effect anendothermic chemical reaction, i.e. a reaction that absorbs chemicalheat. This means that the exhaust gas, for example flowing out of thesteam generator—in the direction of flow of the exhaust gas of the gasturbine—has a temperature downstream from the steam generator, suchthat, in the exhaust gas of the gas turbine downstream from the steamgenerator, i.e. after generation of the steam, there is heat containedin the exhaust gas of the gas turbine. This heat that is contained inthe exhaust gas downstream from the steam generator, or after the steamgenerator, is used to affect the endothermic chemical reaction. For thispurpose, the heat contained in the exhaust gas is supplied to theendothermic chemical reaction, or to educts of the endothermic chemicalreaction.

As a result, at least a portion of the heat supplied to the endothermicchemical reaction is stored in products of the endothermic chemicalreaction, such that a thermochemical accumulator, in particular athermochemical heat accumulator, can be created. The heat contained inthe exhaust gas of the gas turbine downstream from the steam generatorcan be stored, at least partly, in the products of the endothermicchemical reaction, wherein the heat stored in the products can be used,for example, at a subsequent point in time and/or for other purposes.Some embodiments use heat contained in the exhaust gas of the gasturbine after the steam generator, which is usually lost without beingused, for the purpose of storing at least a portion of the heatcontained in the exhaust gas downstream from the steam generator.

In particular, the heat may be stored for district heating purposes. Forexample, an exothermic chemical reaction, i.e. a reaction giving offchemical heat, can be affected, wherein the products of the endothermicchemical reaction are educts of the exothermic chemical reaction, or areused as educts of the exothermic reaction. In the course of theexothermic chemical reaction, heat is released, by means of which amedium, in particular water, can be heated efficiently. Products of theexothermic chemical reaction may be used, for example, as the educts ofthe endothermic reaction.

The thermochemical heat accumulator can be used to realize particularlyhigh flexibility in respect of the realization of district heating. Inparticular, it is possible to store heat, or energy, in thethermochemical heat accumulator, such that, in particular in the case ofhigh demands for heat, a medium, in particular water, can be heatedeffectively by means of the heat stored in the thermochemical heataccumulator. Since energy contained in the exhaust gas downstream fromthe steam generator is used for this purpose, a particularly highefficiency can be realized. The heat that is stored in the products ofthe endothermic reaction, and released in the exothermic reaction, istransferred, for example, to heat the medium. The medium can then beused for heating purposes, in particular to realize district heating.

In some embodiments, at least a portion of the heat contained in theexhaust gas downstream from the steam generator is transferred, via aheat exchanger, to educts of the endothermic chemical reaction.

In some embodiments, at least a portion of the steam generated by meansof the steam generator is branched-off and stored in a steamaccumulator. In addition, at least a portion of the steam stored in thesteam accumulator is removed from the steam accumulator. The steamremoved from the steam accumulator is heated by means of heat that isreleased in the exothermic chemical reaction. In addition, the heatedsteam is routed to the turbine installation, which is driven, inparticular accelerated, by means of the supplied heated steam.

In some embodiments, products of the endothermic chemical reaction areused as educts of the exothermic chemical reaction.

In some embodiments, the heated steam for driving the turbineinstallation is supplied to the turbine installation, to ramp up thegas-and-steam combined-cycle power plant from a first load range into asecond load range that is higher than the first load range. In someembodiments, the endothermic chemical reaction is affected in the secondload range.

Some embodiments may include a gas-and-steam combined-cycle power plantexecuting a method a described above. Advantageous designs of the methodare to be regarded as advantageous designs of the gas-and-steamcombined-cycle power plant, and vice versa.

The single FIGURE, in a schematic representation, shows a gas-and-steamcombined-cycle power plant 10, also referred to as a COGAS power plantor—to improve readability—as a power plant. The power plant 10 comprisesat least one gas turbine 12, to which fuel is supplied, for example inthe course of a process for operating the power plant 10. This supply offuel to the gas turbine 12 is indicated in the FIGURE by a directionarrow 14. The fuel may include a gaseous fuel such as, for example,natural gas. In addition, air is supplied to the gas turbine 12, thisbeing indicated in the FIGURE by a direction arrow 16. The fuel isburned by means of the gas turbine 12, resulting in exhaust gas of thegas turbine 12. The gas turbine 12 thus provides the exhaust gas, asindicated in the FIGURE by a direction arrow 18. A mixture of the fueland the air, for example, is formed in the gas turbine 12, this mixturebeing burned. This results in the exhaust gas of the gas turbine 12.

It can be seen from the direction arrow 18 that the exhaust gas issupplied to a steam generator 20 of the power plant 10. The steamgenerator 20 may be referred to as a boiler or evaporator. In addition,a fluid, e.g. water, is supplied to the steam generator 20. A transferof heat is affected from the exhaust gas of the gas turbine 12 to thewater, as a result of which the water is heated and evaporated. As aresult, steam is generated from the water. This means that, by means ofthe exhaust gas of the gas turbine 12 and by means of the steamgenerator 20, steam is generated from the water (fluid) supplied to thesteam generator 20. As a result of this transfer of heat from theexhaust gas to the water, the exhaust gas is cooled, such that it isremoved from the steam generator 20, for example, at a first temperatureT1. The first temperature T1 is, for example, at least substantially 90°C. (degrees Celsius).

The power plant 10 additionally comprises a turbine installation 22,which in the present case comprises a first turbine 24 and a secondturbine 26. The turbine 24 may comprise a high-pressure turbine andturbine 26 may comprise a medium-pressure and low-pressure turbine. Thesteam generated by means of the exhaust gas of the gas turbine 12 and bymeans of the steam generator 20 is supplied to the turbine installation22, such that the turbine installation 22, in particular the turbines 24and 26, are driven by means of the generated hot steam. By means of theturbine installation 22, energy contained in the hot steam is convertedto mechanical energy, the mechanical energy being provided via a shaft28. The turbine installation 22 comprises, for example, turbine wheels,not represented in detail in the FIGURE, to which the steam is supplied.As a result, the turbine wheels are driven by means of the steam. Theturbine wheels are connected, for example, in a rotationally fixedmanner to the shaft 28, such that the shaft 28 is driven by the turbinewheels when the turbine wheels are driven by means of the steam.

The power plant 10 additionally comprises at least one generator 30,which can be driven, or is driven, by the turbine installation 22, viathe shaft 28. The mechanical energy provided via the shaft 28 is thussupplied to the generator 30, at least a portion of the suppliedmechanical energy being converted to electrical energy, or electriccurrent, by means of the generator 30. The generator 30 can provide thiselectric current, which, for example, can be fed into an electricitygrid.

The steam is removed from the turbine installation 22 and supplied to aheat exchanger 32, which may comprise a condenser. By means of the heatexchanger 32, the steam is cooled, as a result of which the steamcondenses. As a result of this, the steam again becomes theaforementioned water, which can be supplied back to the steam generator20. To cool the steam by means of the heat exchanger 32, a coolingmedium, in particular a cooling fluid, may be supplied to the heatexchanger 32. A transfer of heat can then be affected from the steam tothe cooling fluid, as a result of which the steam is cooled andsubsequently condenses.

The power plant 10 has a plurality of lines, not represented in greaterdetail in the FIGURE, flowing through which there are respective flowsof the steam generated by means of the exhaust gas of the gas turbine12. These flows may have differing temperatures. Represented in theFIGURE are differing temperatures T2, T3, and T4 of the steam generatedby means of the exhaust gas of the gas turbine 12, for example thetemperature T2 being 595° C., the temperature T3 360° C., and thetemperature T4 240° C. The water leaves the condenser, for example, at atemperature T5, which is, for example, 40° C.

Depending on the demand for electricity, the power plant 10 isactivated, i.e. switched on, and/or deactivated, i.e. switched off. Forexample, the power plant 10 is switched off if there is only low demandfor electricity. If the demand for electricity increases, then the powerplant 10, after having been switched off, is switched on again. Thisswitching-on subsequent to switching-off may be a warm start, to enablethe power plant 10 to be switched on in a rapid and energy-efficientmanner. To realize this warm start, in particular to realize aparticularly energy-efficient warm start, the power plant 10, afterhaving been switched off and during a period in which the power plant isswitched off, is kept warm, or heated, in order to avoid excessivecooling off, or cooling down, of the power plant 10.

The gas turbine 12 provides its exhaust gas to the steam generator 20.In addition, the water is supplied to the steam generator 20. By meansof the exhaust gas of the gas turbine 12 supplied to the steamgenerator, and by means of the steam generator 20, the water is heatedand evaporated, at least partly, as a result of which steam isgenerated. In addition, the exhaust gas of the gas turbine 12 that issupplied to the steam generator 20 is removed, at least partly, from thesteam generator 20.

To realize a particularly high efficiency, or particularly efficientoperation, the power plant 10 may comprise a thermochemical heataccumulator 34 comprising at least one reactor. Since the exhaust gas ofthe gas turbine 12—relative to a direction of flow of the exhaust gas ofthe gas turbine 12—downstream from the steam generator 20, i.e. afterthe steam generator 20, has the temperature T1, the exhaust gas of thegas turbine 12 downstream from the steam generator 20 contains heat.

At least a portion of this heat contained in the exhaust gas of the gasturbine 12 downstream from the steam generator 20—as indicated in theFIGURE by a direction arrow 36—is supplied to the thermochemical heataccumulator 34 (reactor). This heat supplied to the thermochemical heataccumulator 34 is used to affect an endothermic chemical reaction. Inother words, an endothermic chemical reaction is affected by means ofthe heat, from the exhaust gas removed from the steam generator 20, thatis supplied to the thermochemical heat accumulator 34. As a result, theheat supplied to the thermochemical heat accumulator 34, or at least aportion of the heat supplied to the thermochemical heat accumulator 34,is stored in products of the endothermic chemical reaction, the storedheat being able to be used according to demand.

At least the portion of the heat contained in the exhaust gas of the gasturbine 12 downstream from the steam generator 20 is supplied to thethermochemical heat accumulator 34, in particular to the endothermicchemical reaction, or educts of the endothermic chemical reaction, forexample via at least one heat exchanger 38, through which at least aportion of the exhaust gas flows. In this case, there is a transfer ofheat from the exhaust gas, via the heat exchanger 38, to educts of theendothermic chemical reaction. Relative to the direction of flow of theexhaust gas, the heat exchanger 38 is arranged downstream from the steamgenerator 20.

As a result of the described transfer of heat, the exhaust gas iscooled. The exhaust gas that is supplied to the heat exchanger 38—asindicated in the FIGURE by a direction arrow 40—is, for example, removedfrom the heat exchanger 38, and downstream from the heat exchanger 38has, for example, a temperature T6 that is 70° C. and less than thetemperature T1. In addition, the exhaust gas may have a mass flow rateof 884 kg/s and a pressure of one bar. Furthermore, at least a portionof the exhaust gas flowing out of the steam generator 20 is supplied tothe heat exchanger 38, or to the thermochemical heat accumulator 34.

The endothermic chemical reaction is, for example, a forward reaction ofa chemical equilibrium reaction. In the course of the forward reaction,products of the endothermic chemical reaction are produced from theeducts of the endothermic chemical reaction (forward reaction). Thischemical equilibrium reaction also comprises a back reaction, realizedas an exothermic chemical reaction. In this case the products of theforward reaction are educts of the back reaction, and products of theback reaction are the educts of the forward reaction. The forwardreaction and/or the back reaction is/are affected, for example, in thereactor, i.e. in the thermochemical heat accumulator 34.

Heat is released in the course of the back reaction. This heat thatbecomes free or is released in the course of the back reaction can beused for heating purposes, in particular for district heating purposes.For example, it is conceivable to use heat released in the course of theback reaction to generate steam, and/or to heat, in particular tosuperheat, provided steam, in order to heat, for example, at least aportion of the power plant by means of the generated, or heated, steam,or alternatively to drive, in particular to accelerate, the turbineinstallation 22, such that, for example, the power plant can beramped-up from a first load range into a second load range that ishigher than the first.

In the present case, however, the heat released in the back reaction isused for heating purposes, in particular district heating purposes. Insome embodiments, a fluid may be heated by means of the heat released inthe back reaction. The water is supplied to a further heat exchanger 42of the thermochemical heat accumulator, as indicated in the FIGURE by adirection arrow 44. The heat released in the back reaction is supplied,via the heat exchanger 42, to the water flowing through the heatexchanger 42, as a result of which the water is heated. The heated wateris removed from the heat exchanger 42, as indicated in the FIGURE by adirection arrow 46. The water has, for example, a mass flow rate of 1100kg/s (kilograms per second). The water is provided at a temperature T7,for example, the water being supplied at the temperature T7 to the heatexchanger 42. By means of the heat exchanger 42, the water is heated toa temperature T8, for example the temperature T7 being 65° C. (degreesCelsius) and the temperature T8 being 100° C. The temperature T8 is thusgreater than the temperature T7, the water having the temperature T7upstream from the heat exchanger 42, and the temperature T8 downstreamfrom the heat exchanger 42. It is additionally provided, for example,that the water has a pressure of 14.5 bar, the water being provided atthis pressure and at the temperature T7, and supplied to the heatexchanger 42.

Since the forward reaction is affected with the exhaust gas at 90° C.,the thermochemical heat accumulator is charged at 90° C. Since the wateris heated, by means of the thermochemical heat accumulator 34, to 130°C., the thermochemical heat accumulator 34 is discharged at 130° C.

The use of the heat exchanger 38 makes it possible to realize a spatialseparation of the educts of the forward reaction from the exhaust gas,such that the exhaust gas does not come into direct contact with theeducts of the forward reaction. Alternatively, it is conceivable thatthe exhaust gas does come into direct contact with the educts of theforward reaction, and in this case flows onto, or around, the educts.The heat exchanger 38, for example, is then omitted. This is alsotransferrable to the back reaction: the use of the heat exchanger 42makes it possible to realize a spatial separation of the educts and/orproducts of the back reaction from the water that is heated by means ofthe released heat, such that the water does not come into direct contactwith the educts and/or products of the back reaction. Alternatively, itis conceivable that the water does come into direct contact with theeducts and/or products of the back reaction, and in this case flowsonto, or around, the educts and/or products. The heat exchanger 42, forexample, is then omitted.

The water heated by means of the thermochemical heat accumulator 34 canbe used, for example, to supply households with hot water, and/or forhousehold heating. As a result, a particularly efficient process overallcan be realized. In addition, it is possible to realize particularlyhigh flexibility of the heat supply. In particular, it is conceivablefor peak loads, or high demands for heat, to be covered in anenergy-efficient manner by means of the thermochemical heat accumulator34, since at least a portion of the energy contained in the exhaust gasdownstream from the steam generator 20 is used, at least indirectly, toheat the water. Depending on the mass flow rate of the exhaust gas andof the water, it is conceivable to supply only a portion of the exhaustgas downstream from the steam generator 20 to the heat exchanger 38,and/or only a portion of the water to the heat exchanger 42, to ensure,in particular, an at least substantially continuous heating of the waterby means of the thermochemical heat accumulator 34.

What is claimed is:
 1. A method for operating a gas-and-steamcombined-cycle power plant, the method comprising: providing exhaust gasfrom a gas turbine to a steam generator; generating steam by means ofthe exhaust gas; driving a generator with the steam via a turbineinstallation to provide an electric current; removing the exhaust gasfrom the steam generator; and using at least a portion of heat containedin the exhaust gas downstream from the steam generator to affect anendothermic chemical reaction.
 2. The method as claimed in claim 1,wherein the at least a portion of the heat contained in the exhaust gasdownstream from the steam generator is transferred via a heat exchangerto educts of the endothermic chemical reaction.
 3. The method as claimedin claim 1, further comprising: branching-off at least some of the steamgenerated by the steam generator and storing the at least some of thesteam in a steam accumulator; removing at least a portion of the steamstored in the steam accumulator from the steam accumulator; heating theat least a portion of the steam removed from the steam accumulator withheat released in an exothermic chemical reaction; and routing the heatedsteam to the turbine installation to drive the turbine installation withthe heated steam.
 4. The method as claimed in claim 3, furthercomprising using products of the endothermic chemical reaction as eductsof the exothermic chemical reaction.
 5. The method as claimed in claim3, further comprising supplying the heated steam to the turbineinstallation to ramp up the gas-and-steam combined-cycle power plantfrom a first load range into a second load range that is higher than thefirst load range.
 6. The method as claimed in claim 5, wherein theendothermic chemical reaction is affected in the second load range. 7.(canceled)